The present invention relates generally to microelectronic assemblies having an integrated circuit die attached to a substrate by a plurality of solder connections, and more specifically relates to a microelectronic assembly having a particulate free underfill material.
Many microelectronic assemblies are manufactured using the well known flip chip on board (FCOB) technique. In FCOB assembly, an integrated circuit die is provided which includes a plurality of bond pads, with each of the bond pads having deposited thereon a solder bump. The die is turned over or flipped and is superimposed over a substrate having a plurality of bond pads, such that each of the solder bumps is aligned with a corresponding one of the bond pads on the substrate. The die and the substrate are then reflow soldered together to form solder connections. The gap that remains between the downwardly facing die face and the upwardly facing substrate face is then filled using any one of a number of known underfill materials.
The underfill material, which typically contains silica or other particulates in a resin binder, serves to encapsulate the solder connections and serves to bond the die to the substrate. The underfill also increases the reliability of the microelectronic assembly during thermal cycling by enhancing the mechanical connection between the die and the substrate. Also, by having a coefficient of thermal expansion (CTE) between the CTE of the die and the CTE of the substrate, the underfill material lessens thermal expansion problems. Further, the particulate filler also enhances the shear strength of the filler material.
For a variety of reasons, it is desirable to manufacture such microelectronic assemblies with smaller and smaller gaps between the die and the substrate (typically termed xe2x80x9cdie standoffxe2x80x9d). However, according to modern assembly methods, the underfill material is typically drawn into the gap using capillary action. Thus, the viscosity of the material is an important consideration. As the gaps gets smaller, the size of the particulate filler begins to interfere with the ability of the underfill material to flow freely into the gap. Although smaller particulate fillers for the underfill material are available, such smaller particulates are very expensive and increase the viscosity of the underfill material to unacceptable levels. High viscosity materials interfere with the capillary action and also hinder the encapsulation of the solder connections. Moreover, as the gap size decreases, the shear strength of the material becomes more important.